This invention relates to a new and novel CRT device for generating a bright color light spot of the type that is particularly useful for projection color television and information display.
Green light radiation for use in projection color television in general is produced by the electron bombardment of a green luminescent terbium-activated phosphor such as a terbium-activated yttrium oxide or oxysulfide phosphor. The terbium-activated phosphor, when excited by electronic bombardment, produces a large amount of the desired fundamental radiation at 544 nm, but also produces significant amounts of troublesome radiations at 490 nm, 586 nm and 620 nm. It has been proposed to eliminate the undesired radiations by positioning in the path of the green radiation a light beam filtering means comprising a light filtering solution held in place by a container, the portions of which in the path of the green radiation are formed of glass transparent to the green radiation.
Methods of achieving these results which have been proved successful at least in part are described in British Patent Application U.K. 2098393A, Kikuchi et al I.E.E.E. Transactions on Consumer Electronics, Vol. CE-27, No. 3, August 1981, pp. 478-484, copending U.S. application Ser. No. 453,379, filed Dec. 27, 1982, now U.S. Pat. No. 4,547,699 and copending U.S. application Ser. No. 548,065 filed Nov. 2, 1983, now U.S. Pat. No. 4,538,089 both filed by an inventor common to the instant application and both commonly assigned.
It has been found however that because of the mismatch between the indices of refraction of the filtering solution and the glass plates forming the walls of the container through which the filtered radiation passes (which glass plate is hereinafter called the "face plate") a "halo" effect is produced around the projected spot due to reflections at the solution/glass interfaces.
It is found that such an undesired halo effect not only is encountered when filtering solutions are employed for eliminating undesired radiation from cathode ray tube devices employed for generating bright green light spots but also when filtering solutions are employed with cathode ray tubes used for generating bright blue or bright red light spots as described for example in copending U.S. application Ser. No. 600,049, filed Apr. 13, 1984, now U.S. Pat. No. 4,572,984 and copending U.S. application Ser. No. 659,103 filed Oct. 9, 1984 both filed by one of the instant inventors and both commonly assigned. In all these cases the cause of the "halo" effect is the same, the mismatch of the indices of refraction at the solution/glass interface.